The Science Thread here

Originally posted by ImaFungi
Gravity is thought to travel at the speed of light, thats very interesting. So maybe the reason light travels at the speed of light...

The reason is because both of these interactions are represented by the exchange of massless particles, and massless particles always travel at the
same speed, which is the maximum possible speed. It's convenient and easy to measure the speed of light, so we always call this maximum speed "the
speed of light."

graviton theory is silly imo, it takes no effort to come up with that theory

This assessment is based on your decades of study into theoretical physics, and extensive math background?

well we think everything that exists are particles or bits

I don't know what "bits" are supposed to mean, but everything has a description in terms of "particles", but when we talk about particles in
modern (i.e., after about the 1900s) physics, we don't mean the 1700s idea of particles. We mean very precisely defined mathematical things. This
description is equivalent to one in terms of "fields" because the modern definition of the two things is the same.

and we dont really understand gravity

You don't. We do.

but thats probably particles too

Its underlying description has a low-energy decomposition in terms of Fourier modes that one interprets as particles, yes.

oh and also nothing is really particles, everythings really waves, jk, everythings really fields, what are fields, well they dont really exist, and
noone knows.

Again, the descriptions have been unified for like a century now. There's really no excuse for not knowing that at this point.

Is there a planet that doesnt rotate? Is there a planet that doesnt rotate with a moon? Is there a moon that doesnt rotate with a moon?

There are none in this solar system that don't rotate, any particular one not rotating is not common because the discs of gas and dust that form them
are rotating. You'd never find a planet that doesn't rotate with a moon, though, since the tidal forces of the moon would cause the planet to start
rotating.

Originally posted by Arbitrageur
Space is a different concept in relativity than in quantum mechanics. They don't really agree on exactly what it is which is why a theory of quantum
gravity would be nice to have.

No, they absolutely aren't! This is why quantum field theory is easily made to be specially relativistic. The difficulty in building quantum gravity
has absolutely nothing to do with this or any other "philosophical" issue. It has to do with how effective field theories work--the low energy
physics is an average over the high energy physics, therefore combining two low-energy theories (quantum field theory and gravity) is not guaranteed
to be well-defined because you have unknowingly written down a description which has averaged out important details.

Systematically determining which details have been averaged out is non-trivial, but were determined by many physicists independently, in the '60s,
'70s, and '80s though many different approaches. In the 80s-90s these ideas were very nicely all incorporated into String Theory / M-Theory.

Originally posted by Moduli
No, they absolutely aren't! This is why quantum field theory is easily made to be specially relativistic. ..In the 80s-90s these ideas were very
nicely all incorporated into String Theory / M-Theory.

Quantum field theory as it is ordinarily done ignores gravity. But as long as one is ignoring gravity, one can add any constant to ones definition
of energy density without changing the predictions for anything you can experimentally measure. The reason is that without measuring the curvature of
spacetime, one can only measure energy differences.

So relativity looks at the curvature of space-time, and quantum field theory ordinarily
ignores it according to Baez.

You said gravity works by the exchange of virtual particles and also mentioned geometries related to gauge theories. How does gravity work by
exchanging virtual particles (for example; the moon orbiting the earth)? Every atom on earth is constantly sending infinite virtual particles, and the
moon is doing the same, and these virtual particles grab on to one another and dont let go? I know its complex and intricate, but is there any way
you can simplify your knowledge of how the moon is held into orbit around earth?

The reason is because both of these interactions are represented by the exchange of massless particles, and massless particles always travel at the
same speed, which is the maximum possible speed. It's convenient and easy to measure the speed of light, so we always call this maximum speed "the
speed of light."

Where do all the atoms get their never ending supply of massless particles from? Or the massless particles are just an affect or reaction of atoms
existing?

What is a field and in what way does it exist? And a particle is when 2 fields interact at a point, or when two geometries of one field interact with
itself? Why are there so many different types of fields, and how are they so unique and strong as themselves (how are they not all sloppily molded
into one clustermuck of a field soup?), how can they all interact while maintaining their own integrity? How out of one perfect sameness, did all the
variety come?

Originally posted by Arbitrageur
Quantum field theory as it is ordinarily done ignores gravity. [..] So relativity looks at the curvature of space-time, and
quantum field theory ordinarily ignores it according to Baez.

Two problems here, first, the highlighted segment: it can be done in a way which does not ignore gravity. Second, relativity doesn't look at
curvature, general relativity does. Ordinarily, QFTs use special relativity only, but the only difference between special and general
relativity is that general relativity applies special relativity to everywhere independently. This adds extra constraints, which are described in
terms of curvature. But it's no trouble at all to amend a QFT to do this, too. The difficulty is, as I said, not a philosophical one, but that one
has combined two theories which do not provide a sufficient amount of information to properly determine their union.

The complete specification of the extra information that one needs to provide is given by string theory (and string theory only, as one can explicitly
check, using the definitions). Indeed, string theory is derived as something that must be true by deducing what extra information is needed--it
is not in any way assumed!

Originally posted by ImaFungi
You said gravity works by the exchange of virtual particles and also mentioned geometries related to gauge theories. How does gravity work by
exchanging virtual particles (for example; the moon orbiting the earth)?

There's not really a simple explanation (well, there is, but the simple explanation is essentially Newton's law of gravitation). But the mechanism
that works is no different than how magnets can be felt to attract or repel each other, in this case, the quantum mechanical description is through
the exchange of virtual photons. This isn't to imply gravity is the same thing as electromagnetism--they are quite different--but the same kind of
math describes both of these cases (as well as many others).

Originally posted by ImaFungi
Where do all the atoms get their never ending supply of massless particles from? Or the massless particles are just an affect or reaction of atoms
existing?

They don't "get" them from anywhere. They are there as a consequence of the fact that they can be there without violating conservation of momentum
or energy.

What is a field and in what way does it exist?

It's a very complicated mathematical thing that can't be explained well without math.

The best explanation of these kinds of things, without using real math or physics, is probably either in one of the popular science books written by
actual physicists, or on Matt Strassler's blog (he's a particle physicist, so he doesn't really study string theory or gravity, but has lots of
nice articles on particles and standard model kind of stuff).

Is string theory actually a theory? My impression is that it is more of a math construct.
Does it make testable predictions? Have any of those predictions been demonstrated?

It's a theory as much as anything else in physics is. And it makes plenty of predictions--you can't be physics without making predictions! And,
necessarily, any prediction must be testable, otherwise it's not a prediction!

The difficulty with string theory is that it is capable of making way too many predictions, and it's not always clear what the right way to set up a
problem is. But that's not a problem with the theory, that's a problem with us being too inexperienced at it. But complaining about that is kind of
like complaining that programming is hard, because you can write any possible program, when you really only want to write the one program you're
interested in!

Of course, any good theory must make lots of predictions. For example, when Newtonian mechanics was written down, it was criticized for being hard to
make predictions with. How are you supposed to know in advance what all the forces acting on something are! It's very hard to deduce the equations of
motion that describe something without understanding how to set problems up in general. Now-a-days, we can do that for Mechanics, but outside of a few
special cases, we don't know how for string theory.

But it's easy enough to make general predictions with it. For example, the mass of the recently detected Higgs is in the range that many string
theory models predict. It makes general predictions about things like supersymmetry, and the relationships between normal standard model particles,
such as masses, CKM matrix elements, etc. Although it's still not known which model exactly corresponds to the Standard Model.

There are other predictions it makes outside of Standard Model like things. Gauge/Gravity duality is an obvious one, where a particular string theory
can be shown to be exactly equivalent to an ordinary quantum field theory that at first sight looks very different, and lives in one fewer dimension.
This is particularly interesting because some of these theories look just like the ones that condensed matter physicists study. So it's possible
there could be applications of string theory in things like solid state devices in the next 5 or 10 years.

But it's easy enough to make general predictions with it. For example, the mass of the recently detected Higgs is in the range that many string
theory models predict.

How does the Higgs have a mass? I thought the Higgs was what was invented because noone knew how particles could have mass. So they are saying the
higgs gives all particles mass, and then it gives itself mass aswell? How does it give itself mass, how isnt the same problem of how particles have
mass subscribed to the higgs itself?

Im also curious about this. It is said all atoms are a nucleus surrounded by electron right? So does that mean all we have ever touched and seen are
electrons? (Is the real answer. the results of those electrons vibrating;light is all weve ever seen or touched?)

So im sitting in my chair right now, and the reason me and the chair arent chemically reacting or touching, is because the electrons in my shorts are
repelling the electrons in the chair, and the electrons in the chair are repelling the electrons in the floor? And the only reason my shorts, the
chair, and the floor, can exist as material is because electrons are used some how in molecular bonds?

I read most of those links, thanks. The concept and realistic interpretation of fields is still hard for me to understand. Are fields just physical
3-d graphs? Or are they imagined how there are different layers of the earth, and in the universe there are different layers (fields) but they all
exist together, and the result of them existing together are particles interacting? So for example, if all that existed was 1 field, would any
particles exist? Or it would just be a web of unquantifiable energy, or relative motion of the field would result in differing values at different
points on/in the field and that would be the /particles? My problem I think is with understanding how the field it self exists as a structure. We
know things like apples have structure, and are composed of parts and quantities and there are boundaries to these parts, and boundaries of the apple
as a whole. But it seems like when one is talking of the existence of a field, they speak as if the field is not the sum of its parts or particles,
but that the particles are results of the field. There are things that hold an apple together, and its atoms together, they are connected in some
physical way. How is a field connected to itself, how is it so large, = to itself, with out constituents. Is there anything you could compare this
to? To me it would be as if the electron, which is thought to have no constituents, was the same size as the universe.

I have heard it said when matter and anti matter collide they 'annihilate into pure energy'. What is the nature of this energy, that they turn
into? What happens to the matter they were? Is the energy they create stable? Why does this reaction take place?

I would also like to ask; Is the reason the electron behaves as it does in the orbital because of forces felt by the electron from the nucleus,
forces felt by the electron from other electrons in the orbital, and forces felt by the electron in the immediate environment surrounding this
atom?

According to Richard Feynman, the reason that the electron doesn't stick onto the nucleus is that, if the electron did stick onto the nucleus we
would know where it was and that would violate the uncertainty principle.

In other words, the quantum theory uses empirical data to make rules specific to the atomic range of physics. No laws of pre QED-physics are assumed
to apply.

I guess that the nucleus makes the orbitals in a way analogous to a mass curving space by the force of gravity or an electric charge setting up and
powering an electric field.

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